During neurological surgery, therapy or diagnosis, it is common practice to work with a reference system being fixed in relation to the head of the patient using some type of fixation device. The reference system is used to determine the location of different areas in the involved part/tissue of the patient.
An example of such a reference system is a so-called stereotactic frame with posts having fixation pins for invasive fixation to the skull of a patient. In use during for example MRI (Magnetic Resonance Imaging) diagnostics, the stereotactic frame is arranged around the head of a patient, and the fixation pins of the posts connected to the frame are screwed to the bone of the skull, thus ensuring a rigid fixation of the reference system.
The frame is then rigidly held in position in relation to a MRI table.
For use in MRI diagnostics, all parts of the stereotactic frame, the posts and pins must be made of non-magnetic material. For the frame and posts, aluminium is often used. For the pins, several different materials have been proposed.
U.S. Pat. No. 5,643,268 (Vilsmeier) discloses fixation pins having a threaded plastic shaft and a separate point of semi-precious stone material. These fixation pins are disposable; used only one and then discarded.
Another type of fixation pins are re-usable fixation pins, that need to be sterilised between each use. Preferably, the pins should be sterilisable in an autoclave, being a practically available sterilisation means in hospitals.
Known re-usable fixation pins are for example made of aluminium with a tip of hard metal. Recently, also fixation pins made of titanium have come into use. The titanium pins have the advantage that they are easier to manufacture than the previously used aluminium—hard metal pins. Titanium pins are also readily sterilisable in common autoclave means. Further, titanium have advantageous strength properties. This is useful, for example in that a titanium pin may be made having an internal structure such as a hexagonal blind bore for seating of a screw driver when adjusting the position of the pin. Internal structures are preferred over external structures, such as external hexagonal seats, since extending structures take up more space.
For the frame posts, aluminium has, as mentioned above, often been used. Also plastic posts are available. However, some of the plastic posts have the disadvantage that they are somewhat more resilient than aluminium, thus risking inadvertent bending of the posts, resulting possibly in inaccurate references being used in surgery or diagnostics. Also, plastic posts have limited durability in that they become weakened by the sterilisation procedures and therefore only can withstand a limited number of sterilisations.
One example of plastic posts is found in U.S. Pat. No. 4,612,930 (Bremer) disclosing posts being made of boron fibre or graphite fibre reinforced plastic.
In view of the above, it is appreciated that many different combinations of known posts and pins may be made, after the choice of the user. However, when using metal posts together with metal pins during MRI diagnostics, it has been found that a volume around the pins may be subject to heating. This effect has been found to be particularly pronounced when using metal posts in combination with titanium pins. Sometimes the heating effect becomes considerable, so that a patient may be affected by the increased temperature.
U.S. Pat. No. 5,961,528 (Birk et al) describes a problem related to a temperature change during MRI diagnosis. A skull pin is suggested which includes a metallic tip section being attached to a rigid, non-metallic insulator, preferably ceramic. The proposed skull pin includes a contact tip, an insulator attached to the tip and extending outwardly therefrom, and a driving portion coupled to the insulator for attachment with the cervical fixation device. Thus, although U.S. Pat. No. 5,961,528 recognises a heating problem, the solution consists of a pin comprising several details of different materials.
Therefore, it is an object of the present invention to provide a fixation device, for which said heating of the pins during MRI diagnostics is diminished in relation to previously known devices, and which enables use of metal posts in combination with conventional metal pins.